Chemical castration is the administration of medication designed to reduce libido and sexual activity. … Chemical castration is generally considered reversible when treatment is discontinued. [The irreversibility consists of possible permanent changes in bone density, not in libido and sexual activity]

This explanation gives us the reason why this will never work. Three months after the an injection is given, the police will have to produce the offender to get his next shot. Also, even if the police does manage to produce the person, the quality of the injected drug will have to be monitored. In fact, it is likely that introducing chemical castration will produce two sources of corruption: one in the quality and use of the chemicals themselves, the second in the monitoring of the location of the offender.

Given the record of the police in recording or investigating sex crimes, new legislation should concentrate on implementation rather than punishment. This is an old ploy of political parties: pretend to be strict on crime by increasing penalties, but undermine the process by making sure that implementation of laws remains weak.